SAT identifying sentence errors part 2- best answer gets 10?

Question

the error is in quotes, but I dont understand why it is an error. Explanations plz.

1.Air pollution caused by industrial fumes has been studied for years, but only recently "has" the harmful effects of noise pollution become known.

Why does "has" need to be changed to have in the first place? And if it has to be changed why isnt "has been studied" also an errror?

Sentence two: No matter how "cautious" snowmobiles are driven, they are capable of damaging land.

No idea why cautious is an error. Absolutly no idea...

Answer 1

1. Have - goes with effects.  Has goes with pollution. (plural/sing.)

2. Change to carefully. Snowm. aren't driven cautious, they are driven carefully.

Answer 2

Subject of second part of compound sentence is "effects" which is plural; needs plural verb, "have."

Need an adverb - cautiously - to modify verb are driven. How are snowmobiles driven? Cautiously.  

Answer 3

my guess

air pollution is singular so it is has

harmfull effects is plural so it is have